Pump



United States Patent New York Air Brake Company, a corporation of New Jersey Filed A r. 3, 1963, Ser. No. 270,211 1 Claim. (Cl. 230-158) This invention relates to vacuum pumps of the rotary piston type. Particularly, the invention relates to the balancing of a triplex rotary piston pump.

As known in the art, a rotary piston pump includes a housing having a circular right cylindrical pumping chamber therein. A shaft is journalled in suitable bearings carried by the housing and extends coaxxially through the pumping chamber. A circular cam is eccentrically mounted on the shaft in the pumping chamber. A strap encircles the cam, A valve piston extends radially outward from the strap and passes through a guideway formed in an oscillatory pin journalled in the housing. The valve piston is hollow and has an opening through one side face near the strap. This provides an inlet to the pumping chamber. An outlet connection extends through the housing from the pumping chamber. The inner end of the outlet connection is located near the valve piston on the side opposite the lateral opening therein.

In the past, both simplex and duplex rotary piston pumps have been manufactured. The rotary speeds of such pumps have been limited by the characteristic imbalance of such pumps. According to the prior art, attempts have been made to improve the balance of such pumps by the use of counterweights mounted for rotation with the shaft and careful correlation of the weights of the cams and the counterweights and their positions. Such balancing schemes have not proven satisfactory because the resulting bearing reaction forces were unbalanced causing large shaft deflections unless a large diameter shaft is used. Such schemes also overlook the mass and the true nature of the motion of the valve pins and straps, the problem being treated as one of rotary unbalance. Also ignored are gravity forces and the unbalanced pressure forces on opposite sides of the valve piston and strap assembly.

Because of the unsatisfactory nature of known balancing methods, it has been necessary to make large increases in pump size to achieve commensurate increases in pumping speed.

Applicant has made it possible by practice of his novel balancing scheme to increase rotary speed twofold in all rotary piston pumps except small sized units in which the rotary speed is increased threefold.

According to applicants teaching, the pump is constructed as a triplex unit. Three pumping units, each comprising a cam, a strap, a valve piston and a guide pin, are mounted in axially spaced relation on a single shaft. The mass of the cam, strap and valve piston, respectively, of each end unit is equal to half the mass of the corresponding parts of the intermediate pump unit. The distribution of mass of the cam, strap, and valve piston, respectively, of each unit is the same as that of the same parts in all other units. The center of mass of the moving parts of each pump unit is located on a transverse median plane passing through that unit normal to the shaft. The centers of mass of the cams, and valve pistons, respectively, of one end unit are longitudinally aligned with the centers of mass of the corresponding parts of the other end unit and angularly displaced 180 from the centers of mass of the corresponding parts of the intermediate pump unit. Preferably, but not necessarily, the guide pins of all three units are substantially aligned. Even if the pins are not aligned, all must lie on a single plane passing through the shaft 3,113,5h5 Patented Mar. 16, 1965 axis. The masses of the guide pins of the end units are equal and each has a mass one half that of the pin of the intermediate unit.

The intermediate unit is preferably a single unit, but it may comprise two units, each identical to the end units. The cams may be hollow or solid. The hollow cam is preferred because the pump can start up Without excessive power consumption. The damping effect achieved through use of solid cams will give better damping of upper harmonics of the vibration and they may be used for that reason.

The preferred embodiment of the pump will be described having reference to the accompanying drawing in which:

FIG. 1 is a sectional view on line 11 of FIG. 2.

FIG. 2 is a sectional view on line 22 of FIG. 1.

FIG. 3 is a diagrammatic view showing the phase relationship of the cams.

The pump includes a housing 11, which includes a main body portion 12 having a transverse partition 13 formed integrally therewith, a removable partition 14 mounted therein and end covers 15 and 16 mounted therein. The partitions and end covers define between them pumping units 17, 18, and 19. All three pumping units 1-7, 18, and

19 are mounted on a single shaft 21 journalled in suitable bearings in the end covers 15 and 16 and in the transverse partitions 13 and 14. The pumping units 17, '18 and 19 include the same parts, though unit 18 has substantially twice the axial length of unit 17 (or 19). Pumping unit 19 is shown in detail in FIG. 1. It includes a right circular cylindrical pumping chamber 22 coaxial with shaft 21. A circular cam 23 is mounted eccentrically on shaft 21. An annular strap 24 encircles the cam and has its inner surface mounted with a good running fit on cam 23. The diameter of the cam is chosen so that at the point of maximum eccentricity the outer periphery of the strap is closely fitted to the inner surface of the pumping chamber 22. A valve piston 25 projects upward from the strap 24 and is guided in a guideway formed in cylindrical guide pin 26. The guide pin 26 is journalled in bearing recesses formed in end wall '16 and partition 14 (see FIG. 2). The valve piston 25 has a cored opening 27 extending into it from its upper end. An opening 28 is formed in the side wall of the valve piston near the strap 24. The suction connection is shown at 29 and the discharge connection appears at 31. A spring biased valve 32 prevents back flow through the discharge connection, A water jacket 33 is formed in the housing and has inlet and outlet connections 34 and 35.

The same parts of pumping units '17 and 18 are indicated in :FIG. 2 by the same reference numerals used in describing pumping unit 19 except that respectively and 200 have been added to these reference numerals.

Referring now to FIG. 3, it will be seen that cams 23 and 123 are mounted in phase with each other, i.e., the directions of eccentricity are the same, whereas cam 223 is mounted out of phase with cams 23 and '123, i.e., the direction of eccentricity of cam 223 is opposite to that of cams 23 and 123. Thus, as shown in FIG. 2, pumping units 17 and 19 are at top dead center position, whereas pumping unit 18 is at bottom dead center.

In FIG. 2, the space or system to be evacuated is shown schematically and identified by reference numeral 36. As shown, this space 36 may be connected in parallel with the suction connections 29, 129 and 229 of the pumping units 17, 18 and 19 or the pump may be operated as a multi-stage pump with the units 17 and 18 connected in panal lel fiow relation between the space 36 and the inlet 29 of the unit 19. The size relationship of the units gives a desirable staging ratio of 3:1, whereby the first stage units 17 and 18 and the second stage unit 19 have substanti-ally equal mass rates of flow, i.e., pumping speeds ,3 though the volume of the, pumping chambers in the lower and upper stages differ. in a ratio of 3:1.

The cam 123 has a mass equal to that of cam '23. Both cams are hollow and the mass of each is identically distributed, The cam 223. has a mass twice-that of cams 23 and123. Also, it substantially twice. as long as cams 23 and 123, and-has substantially the same diamcam 223. is slightly smaller indiarneter and slightly more than twice as long ascam 23v or1 23. Pumping chamber 222 is similarly proportioned, This difference in they diameters of the pumping chambers 22 and 222 provides a shoulder by which removable partition 14 maybe precisely positioned normal to shafit 21. In order to distribute the mass of earn 223 in the same way asthe masses of cams 23 and 123 are distributed, the end walls and the central axial web of cam 2231are twice as thick I asthose of the end walls and webs of cams 23. and 123. The cylindrical peripheral walls of. all three cams are substantially equal, in thickness.

The centers of mass of cams 23, 123 and 223 lie on a single plane passing through the shaft axis, the centerot mass of cam 223 dying on the, opposite'side of theshaft rorn. those of dams :23 and 123. This plane also passes through 'the geometric centers of the cams which are similarly arnanged. Thefhollow cams are so designed thatthe center of mass and geometric center. of each cam are approximately coincident. I I

The guide! pins 26, 126, and 225 are shown axially aligned with one another. With this arrangement, the pumping action of the end units is 180. out of. phase with that of the intermediate unit. The pin 226 and the valve piston'225 f the intermediate unit can lie on the eter.- Actually, in the illustrated preferred embodiment,

The triplex pump ofthe present invention can be operated at rotary speeds of over 900 r.p.m. This contrasts favorably with maximum rotary speeds of 360 rpm. and 450 rpm. achieved respectively in commercially available simplex and duplex rotary vacuum pumps.

'In small sizes, such as 5 c.f.m., duplex pumps have been operated at speeds up to'600 11-p n. A pump constructed in accordance-with the. present invention and having a volumetric displacement equal to that of the 5 c.f.m.' duplex pump can be operatedat speeds up to 1800;r.p.m.'

and a pumping speed of 15 c.f.m. is -achieved. 1

The balance of the pumps built in accordance. with this invention is so good that it is not necessary-tobolt their supporting mounts as was the case with the prior simplex and duplex pumps. 6

The inventionis not limited to use with the structure of the illustrated pump, and no such limitation: should be 4 inferred except as expressly recited in. the appended claim:

What I claim is: A vacuum pump comprising: (a) shaft means; 6 (b) three pumping units spaced along the shaft means and driven thereby, the end units operating in phase with one anotherand out. of phase with the intermediate unit; I I (0) each pumping unit consisting of a rotary piston pump and including:. I

(1) a housing having a close-ended rightcircular cylindrical pumping chamber therein concentric wi-thsaid shaft means, (2) a right circular cylindrical cameccentrically mounted onithe shaft" means injsaid chamber, (3) a strap journ alled on the cam and having a right circular cylindricalouter peripheral surface, and having its element of maximum eccentricityin sealing contact, with the. cylindrical wail of the pumping chamber, (4) a rotary pin, whose. axis is parallel with. the 6 of the, shaft'means, journalled in the housing adjacent the pumping'chamber and having a'transverse guideway extending therethrough,

and (5) }a valve piston pnojectingoutward fromv the strap through said guideway; .(d). in which'the mass ofthe cam, strap, and piston, respectively, of one. and unit isequal to the mass of the same parts in the. other end unit and equal to one-half the mass of the corresponding parts of the intermediate unit; a

(e) in which the cam, strap, and valve piston of one unit have a mass distribution about the axis. of the cam, which is the same as the mass distribution-of the corresponding parts of the other unitsabout the corresponding axis; and

(f) in :which'the center of masssof the cam and strap a of the intermediate pump unit'are oppositely displaced from theaxis off the shaft means. with respect to the displacement of the. centers iofmass of the cams and, straps of the end pump units;

(g) in which-:the, axis of the. pins of. bothend pump units and-.of therin'termediate pump unit lie in a plane passing lit-rough the axis of the shaft; and

(Ii) the. centers of mass of each pumpunit lie. on a median plane of the pump unit normal to the axis of the shaft.

References 'Citedin the fileof this patent UNITED STATES PATENTS i 3,107,047 Lorenz Oct. 15, 1963 FOREIGN PATENTS 1,23 ,343 France June 7, 1960 531,190 Italy July 22, .1955 

